Structurally supporting insert for spinal fusion cage

ABSTRACT

An expandable implant includes a structural insert to provide a robust connection between an insertion instrument and the expandable implant. The structural insert can be made from a different material than the remainder of the implant to withstand compressive, tensile, shear, and torsional loads which may be present while inserting the implant into a patient. The structural insert may be formed as part of a bottom member of the implant or may be a separate element inserted into the implant body. The structural insert may provide a threaded connection to an insertion instrument. The expandable implant may include a bone graft port in fluid communication with a bone graft opening extending through the implant body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/554,684, filed Nov. 26, 2014, which claims benefit of U.S.Provisional Patent Application No. 61/909,667, filed Nov. 27, 2013, thedisclosures of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention generally relates to medical devices forstabilizing the vertebral motion segment. More particularly, the presentinvention relates to a composite spinal intervertebral body cage fordistraction and fusion.

Certain known spine cages or implants are characterized by a bodycomprising a hydroxyapetite coated surface provided on the exteriorsurface for contact with adjacent vertebral segments or endplates. Acage of this type may be inserted posteriorly through the neuroforamenof the distracted spine after a surgeon removes disc, bone, and ligamentmaterial to create a pathway.

Such existing devices for interbody stabilization have important andsignificant limitations. Current devices for interbody stabilizationinclude static spacers composed of titanium, PEEK, and high performancethermoplastic polymer produced by VICTREX, (Victrex USA Inc, 3A CaledonCourt; Greenville, S.C. 29615), carbon fiber, or resorbable polymers.

One problem with conventional devices for interbody stabilization madeof PEEK, other high performance thermoplastics or resorbable polymers isthe relative weakness and/or brittleness of these materials compared tothe forces required to insert the device between bones of the spinalcolumn. A review of the Food and Drug Administration's Medical DeviceReporting (MDR) database for intervertebral body cages show that thegreatest reported failure rate, at 36% of all reports, is for breakageof the cage during insertion. Therefore there is a need forintervertebral body cages made from materials that can withstand theinsertion forces without breaking.

The failure point for most cages experiencing breakage during insertionis the point of attachment between the intervertebral body cage and theinserter attached to the cage which is used to place the cage betweenthe vertebrae. There are many means know to those skilled in the art forattaching a spinal fusion cage to an insertion instrument, including,but not limited to a threaded hole and threaded screw, an impression orindentation and hooks or projections, and a supporting surface and aclamping mechanism. In all cases, the attaching means must not onlysecure the spinal fusion cage to the inserter and then release the cageonce it is properly located in the intervertebral space, but theattaching means must also provide a secure attachment during theinsertion step when significant forces may be required to advance thecage between vertebral bodies that have come in contact or near contactaround a “collapsed” disc space.

Impact loads of greater than 50 pounds force have been measured duringthe insertion of intervertebral spinal cages between vertebrae. Evenmore challenging can be the rotational moments placed on the implant asit is forced into a rigidly defined space as more than 90 inch-pounds oftorque have been recorded during insertion. Therefore there is a needfor intervertebral body cages with robust insertion attachment which canwithstand the insertion forces without separation.

BRIEF SUMMARY OF THE INVENTION

An expandable implant according to one aspect of the disclosurepreferably comprises a body having an attachment port and a bone graftport, a top member moveable with respect to the body, and a structuralinsert positioned at least partially within the body and configured tocouple to an insertion instrument, wherein the structural insert is madefrom a different material than the body.

An expandable implant according to another aspect of the disclosurecomprises a body having an attachment port and a bone graft port, a topmember, a bottom member, and a structural insert coupled to the bottommember and configured to couple to an insertion instrument, wherein thestructural insert is made from a different material than the body.

The body may be constructed of a polymer and the structural insertconstructed of metal. The body could also be composed of PEEK and theinsert could be one of titanium alloy, stainless steel alloy, and cobaltchromium alloy.

An expandable implant can be configured to expand hydraulically. Thebody may have a bone graft opening extending through the top member andbody, wherein the bone graft port is in fluid communication with thebone graft opening. An expandable implant can also have a torqueresistant port formed in the body configured to couple to a tab on aninsertion instrument to prevent the body from rotating relative to theinsertion instrument. In at least one embodiment, the structural insertcan provide a threaded connection with an insertion instrument. Theattachment port may have a smooth surface and be concentric with athreaded opening of the structural insert. The body can have an openinginto which the structural insert is placed.

A method of inserting an expandable implant according to one aspect ofthe disclosure comprises providing an expandable implant having a topmember and a body, wherein the implant is expandable from a first,contracted state to a second, expanded state, coupling an insertioninstrument to the expandable implant by extending the instrument throughan attachment port and into a structural insert made from a differentmaterial than the body, inserting the expandable implant through anincision, and expanding the implant.

The expanding step preferably includes expanding the top member awayfrom the body via hydraulic fluid. The coupling step may includecoupling an insertion instrument to the structural insert by threading athreaded end of the insertion instrument into a threaded opening in thestructural insert.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the subject matter of the presentinvention and the various advantages thereof can be realized byreference to the following detailed description, in which reference ismade to the accompanying drawings:

FIG. 1 is a perspective view of an embodiment of the invention.

FIG. 2 is a top view of the embodiment in FIG. 1.

FIG. 3 is a partial cross-sectional view through Line A-A of theembodiment in FIG. 2.

FIG. 4 is a partially exploded perspective view of the embodiment inFIG. 1.

FIG. 5 is a partially exploded perspective view of an alternativeembodiment of the invention.

FIG. 6 is a perspective view of an embodiment in FIG. 5.

DETAILED DESCRIPTION

In exemplary embodiments, the present disclosure is directed to a devicefor providing spinal support for fusion wherein the device contains astructural insert to support the loads placed on the device duringinsertion.

FIG. 1 shows an embodiment of a spinal fusion cage 10 including a topsurface 12, a bottom surface 14, a distal face 16 and a proximal surface18. The proximal surface 18 is configured to contain an attachment port20, a torque resistant port 22, a fluid port 24 and a bone graft port26. The attachment port is used as a means for attaching the spinalfusion cage 10 to an insertion instrument (not shown) for placing thespinal fusion cage into the prepared intervertebral space.

In this exemplary embodiment, the attachment port 20 is a circularopening that is in communication with a structural threaded insert 30(best shown in FIGS. 3 and 4). The structural threaded insert iscomprised of a material that is typically stronger that the material ofthe body of the spinal fusion cage. For example, if the body of theimplant is made of a material such as PEEK or other biocompatiblepolymer, the structural threaded implant 30 can be made from a metalsuch as a titanium alloy, a stainless steel alloy, a cobalt chromiumalloy, or other suitable, biocompatible high strength materials as willbe appreciated by persons of ordinary skill in the art. In this mannerthe structural threaded insert 30 is configured to withstand greaterinsertion forces placed on the spinal fusion cage 10 and thus lessen thepossibility that the threaded connection for the insertion tool or thespinal fusion cage 10 itself will fail.

The fluid port 24 is configured to accept expansion fluid into thespinal fusion cage 10 when the spinal fusion cage is configured toexpand hydraulically. The bone graft port 26 is configured to accept abone graft or bone ingrowth promoting substances such as a demineralizedbone matrix, the patient's own autogenous bone or cadaveric allograftbone, and direct the substance into the central bone graft opening 28.

When a structural insert 30 is provided as is shown in this exemplaryembodiment, there may be a need for a torque resistant feature to helpprevent rotational forces placed on the spinal fusion cage 10 fromunthreading the inserter from the spinal fusion cage 10. The torqueresistant port 22 as shown can be a slot or other recess configured toaccept a mating torque supporting projecting tab on the inserter.Alternately, the fluid port 24 or the bone graft port 26 can beconfigured to accept projecting tabs from the inserter.

FIGS. 3 and 4 show how the structural threaded insert 30 is placedinside the spinal fusion cage 10. The structural threaded insert 30 maybe fit into an opening 40 on the bottom surface 14 of the spinal fusioncage 10. It can be seen that the attachment port 20 of the spinal fusioncage 10 is a smooth wall that does not have threads. When attached tothe inserter the structural threaded insert 30 and inserter produce acompressive load on the spinal fusion cage 10. This is desirable as thepolymer material of the spinal fusion cage 10 is much stronger under thecompression loads than it is under tension loads that would occur duringinsertion if the inserter were to be threaded directly into the polymer.

FIGS. 5 and 6 show an alternative exemplary embodiment of a spinalfusion cage 110, including a top surface 112, a bottom surface 114, andan attachment port 120. In this embodiment, the bottom surface 114 formsthe base of the structural threaded insert 130, and also include one ormore supporting tabs 116 a-c. The spinal fusion cage 110 has an opening140, which is configured to contain both the structural threaded insert130 as well as the supporting tabs 116 a-b and the bottom surface 114.The addition of the bottom surface 114 and the supporting tabs 116 a-cdistribute the insertion loads placed on the spinal fusion cage 110 overa greater area and further reduce the percentage of spinal fusion cagesthat would experience breaks during insertion.

Exemplary embodiments described herein are particularly well suited tobe employed with selectively extendable implants such as disclosed, forexample, in U.S. patent application Ser. No. 12/787,281, filed May 5,2010, entitled “Adjustable Distraction Cage With Linked LockingMechanisms,” the disclosure of which is incorporated herein by referencein its entirety.

For instance, FIG. 3 shows a cylinder 32 configured to receive a piston(not shown). The spinal fusion cage 10 could comprise any number ofcylinders (e.g. two, three, four) although only one cylinder is shown.The cylinder is pressurized by introducing a fluid through the fluidport 24 and into the cylinder 32. When the cylinder 32 is pressurized,the pistons are displaced, translating the top surface 12 away from thebody 34, thereby expanding the spinal fusion cage 10. The fluid can be,for example, hydraulic fluid. It is contemplated to include mechanismsassociated with the cylinder and piston arrangement to maintain theirdisplacement, such as upper lock supports, lower lock supports, and alocking actuator. The upper and lower lock supports may have an invertedstair case and upright staircase configuration, respectively. As shownin FIG. 3, the portion of the cylinder 32 closer to the bottom surface14 illustrates one configuration of an upper lock support. The lockingactuator may be a spring for example which rotates the lower locksupport relative to the upper lock support when the spinal fusion cage10 is expanded. The lower lock support engages the upper lock support asit is rotated by the locking actuator so as to lock the spinal fusioncage in an expanded configuration.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

The invention claimed is:
 1. A spinal fusion cage, comprising: a bodyhaving a first surface for abutting a first vertebral body, an opposingsecond surface for abutting a second vertebral body, and a proximalsurface extending transverse to the first and second surfaces, theproximal surface including an attachment port for receiving a threadedportion of an insertion instrument; and a structural insert made from adifferent material than the body, the structural insert being positionedwithin the body and spaced distally from the proximal surface, such thatat least a portion of the body is defined between the structural insertand the proximal surface, the structural insert having a predefinedthreaded connection configured to detachably and rigidly couple to thethreaded portion of the insertion instrument when the threaded portionis received within the attachment port, such that, when the structuralinsert is coupled to the insertion instrument, the orientation of thebody of the implant is rigidly fixed with respect to the insertioninstrument and a distal end of the insertion instrument does not exitthe implant through the first surface or the second surface.
 2. Thespinal fusion cage of claim 1, wherein the body is constructed of apolymer and the structural insert is constructed of a metal.
 3. Thespinal fusion cage of claim 2, wherein the body is PEEK and thestructural insert is one of titanium alloy, stainless steel alloy, andcobalt chromium alloy.
 4. The spinal fusion cage of claim 1, wherein thebody is configured to expand by displacing the first surface away fromthe second surface.
 5. The spinal fusion cage of claim 4, wherein thebody is configured to expand hydraulically.
 6. The spinal fusion cage ofclaim 5, wherein the proximal surface includes a fluid port configuredfor introducing a fluid for expanding the body.
 7. The spinal fusioncage of claim 1, wherein the proximal surface includes a bone graftport.
 8. The spinal fusion cage of claim 7, further comprising a bonegraft opening extending through the first and second surfaces of thebody, wherein the bone graft port is in fluid communication with thebone graft opening.
 9. The spinal fusion cage of claim 1, furthercomprising a torque resistant port formed in the body and configured tocouple to a tab on an insertion instrument to prevent the body fromrotating relative to the insertion instrument.
 10. The spinal fusioncage of claim 9, wherein the torque resistant port is formed in theproximal surface of the body.
 11. The spinal fusion cage of claim 1,wherein the attachment port has a smooth surface and is concentric withthe threaded connection of the structural insert.
 12. The spinal fusioncage of claim 1, wherein the body has an opening into which thestructural insert is retained.
 13. The spinal fusion cage of claim 12,wherein the opening for retaining the structural insert is in the secondsurface of the body.
 14. The spinal fusion cage of claim 1, wherein thesecond surface is defined on a bottom member of the body, and whereinthe bottom member forms a base of the structural insert.
 15. The spinalfusion cage of claim 14, wherein the bottom member includes at least onesupporting tab extending into the body.
 16. The spinal fusion cage ofclaim 1, wherein the structural insert is configured to couple to thethreaded portion of the insertion instrument such that the threadedportion extends perpendicular to the proximal surface of the body. 17.The spinal fusion cage of claim 1, wherein the body extendslongitudinally along an axis, and wherein the structural insert isconfigured to couple to the threaded portion of the insertion instrumentsuch that the threaded portion extends parallel to the axis.